1. Field of the Invention
The present invention relates to a load cell, and more particularly to load cell of small size and high precision.
2. Description of the Related Art
A load cell which is constituted by providing a load application section at the center of the load cell, providing an annular fixed section around the load application section, connecting the load application section with the fixed section by means of arms, and forming holes in the arms so as to form a Roberval mechanism section has been known.
FIG. 3 shows an example of a conventional load cell. A load application section 1 to which a load is applied is provided at the center of the example load cell, an annular fixed section 2 (or a frame section) is formed around the load application section 1, and the load application section 1 and the fixed section 2 are connected by three pieces of three-way arms 3 of symmetry. A hole section 4 is formed at each of the arms 3. At the positions (a distortable section or a flexure section) of an upper surface and a lower surface of each arm 3 which correspond to the hole section, a pair of distortion gauges 5 (a total of four components: two on the upper surface and two on the lower surface) are installed.
With such constitution, when a load W is applied to the load application section 1, a distortion corresponding to the load W is produced in the distortable section, and the mass of the applied load W can be measured by measuring the distortion using the pair of gauges 5.
However, when the load W is applied to the load application section 1, in addition to the distortion corresponding to the load W which arises in the distortion vulnerable section, an error in output of the gauge 5 arises because of the influence of tension within the arms 3, thereby causing a problem such that it is often impossible to obtain measurements of the desired precision.
Further, because tension can permanently distort the distortable sections, there has been a problem such that it applications involving a heavy load have not been possible.
In order to solve such problems, for example, Japanese Patent No. 2962703 has proposed the constitution such that a flexure section is installed at an upper part of a fixed section in a direction which is perpendicular to a direction of extension of an arm, such that the flexure section absorbs tension.
FIG. 4 shows a load cell having such a constitution. An annular fixed section (a frame section) 13 is formed around a cylindrical load application section 12, and radial arms 14 are connected between the load application section 12 and the fixed section 13. One end of each of the arms 14 is directly connected with the load application section 12, and the other end of each of the arms 14 is connected via each of flexible sections 15 provided on the fixed section 13. Each of the flexible sections 15 is arranged on the fixed section 13 in such a manner that the flexible section 15 lies at a right angle to the arm 14, and each flexible section 15 and the fixed section 13 are united through lower parts of both ends of each flexible section 15.
A hole section 16 is formed in each of the arms 14, and a distortion gauge 17 is inserted into each of the upper and lower parts of the hole section 16.
By connecting the arms 14 to the fixed section 13 via the flexible sections 15 as described above, tension which is created when a load is applied is absorbed by deformation of the flexible sections 15 and the gauges 17 can detect just the distortion which arises due to the load.
However, in the constitution shown in FIG. 4, because the flexible section 15 is installed at an upper part of the fixed section 13 and the arm 14 is fixed to the flexible section 15, there has been a problem such that a height of the entire load cell increases.
Further, from a viewpoint of miniaturization of the load cell as a whole, it is necessary to shorten the radial arms 14 which extend from the load application section 12 and therefore there has been a problem such that it is difficult to improve the precision of such load cells.
The present invention is directed to provide a load cell which is low height, small, and highly precise.
According to one aspect, a load cell according to the present invention comprises a fixed section; a load application section to which a load is applied; an arm section for connecting the fixed section with the load application section and having a distortable section which deforms, when a load is applied to the load application section, according to the load; and a distortion detecting section for detecting distortion which arises in the distortion vulnerable section. Within such an arrangement in which a direction of extension of the distortion vulnerable section is at a specific angle with a straight line which connects the center of the load application section and the center of the distortion vulnerable section, it is possible to restrain tension created when a load is applied to the load application section from being conveyed to the distortion vulnerable section and to secure the linearity of the load and the distortion by controlling an error which may occur due to the tension. In the present invention, because it is not necessary to install a flexible section at an upper part of the fixed section, the height of the load cell can be reduced.
According to another aspect of the present invention, the distortion vulnerable section extends in such a manner that the distortion vulnerable section is substantially perpendicular to the straight line which connects the center of the load application section and the center of the distortion vulnerable section. The maximum restraint of the tension, which arises when a load is applied to the load application section, from being conveyed to the distortion vulnerable section is achieved when the direction of extension of the distortion vulnerable section is at right angles with the straight line which connects the center of the load application section and the center of the distortion vulnerable section.
Further, according to another aspect of the present invention, there is provided a pin to be inserted into a hole which extends through a side surface of the fixed section and substantially reaches the arm section or the load application section perpendicularly to a direction of applying a load. When the pin is inserted into the hole, shearing or failure of the arm section, which may occur when an overload is applied, is prevented. In other words, the pin functioning as a type of beam restrains the arm section in a direction of the application at the time of applying the load to less than a designated value. The maximum permissible flexure of the arm section is determined according to the diameter of the pin.
The present invention will more clearly be understood by referring to the following embodiments.